Refrigerant Circuit

ABSTRACT

The present invention relates to a refrigerant circuit ( 1 ), in particular for use in a liquefaction plant, the refrigerant circuit ( 1 ) at least comprising: —a refrigerator ( 2 ) having an inlet ( 21 ) for refrigerant ( 10 ) at a refrigeration pressure, and at least five outlets ( 22, 23, 24, 25, 26 , . . . ) for evaporated refrigerant ( 20, 30, 40, 50, 60 , . . . ) evaporated at different pressure levels, the at least five outlets ( 22, 23, 24, 25, 26 , . . . ) being preferably intended for refrigerants evaporated at increasing pressures from the first outlet ( 22 ) to the fifth ( 26 ) and optional higher outlets; —a first compressor ( 3 ) having one or more inlets for receiving evaporated refrigerant from the refrigerator and an outlet ( 34 ) that can be connected to the inlet ( 21 ) of the refrigerator ( 2 ); and—a second compressor ( 4 ) having one or more inlets for receiving evaporated refrigerant from the refrigerator and an outlet ( 44 ) that can be connected to the inlet ( 21 ) of the refrigerator ( 2 ).

The present invention relates to a refrigerant circuit, in particular for use in a liquefaction plant.

From practice several line-ups for a refrigerant circuit are known. Usually, a refrigerant circuit comprises a refrigerator (or ‘refrigeration zone’) in which the refrigerant is evaporated in one or more stages thereby withdrawing heat from the gas stream to be cooled; a compressor for recompressing the evaporated refrigerant(s); and return lines for returning the recompressed refrigerant to the refrigerator.

The amount of cooling provided per unit of time in the refrigerator is proportional to the mass flow rate of the refrigerant that is passed through the refrigerator in the refrigerant circuit. With increasing amounts of a stream to be cooled (such as natural gas to be liquefied) the mass flow rate of the refrigerant has to increase. Although an increasing mass flow rate does not affect the number of impellers being present in the compressor, it has an effect on the size of the impellers, on the diameter of the housing, and on the inlet velocity into the impellers. Because the latter variables increase with increasing flow rate, an increasing flow rate will result in a larger compressor and higher inlet velocities. Moreover, increasing the diameter of the housing of the compressor requires a thicker wall of the housing. Consequently the compressor is more difficult to manufacture and more difficult to handle.

As plants for the liquefaction of natural gas and other gas processing plants are being designed for ever-increasing production rates in order to realize the favourable economic benefits associated with larger plants, there exists a continuing need in the field to provide alternative plants and methods to eliminate the size and inlet velocity of a single large compressor

To this end WO 01/44734 (which is hereby incorporated by reference) discloses a refrigerant circuit for use in a liquefaction plant, wherein the refrigerant circuit contains a compressing apparatus with two compressors, each compressor being arranged in a separate housing. The compressing apparatus according to WO 01/44734 allows the handling of four different refrigerant streams being evaporated in a refrigerator at multiple pressure levels.

U.S. Pat. No. 3,527,059 and U.S. Pat. No. 6,691,531 disclose refrigerant circuits allowing to handle refrigerant streams evaporated at three different pressure levels.

It is an object of the present invention to fulfil the above need and to provide an alternative refrigerant circuit.

The above or other objects can be achieved according to the present invention by providing a refrigerant circuit, in particular for use in a liquefaction plant, the refrigerant circuit at least comprising:

-   -   a refrigerator having an inlet for refrigerant at a         refrigeration pressure, and at least five outlets for evaporated         refrigerant evaporated at different pressure levels;     -   a first compressor having one or more inlets for receiving         evaporated refrigerant from the refrigerator and an outlet that         can be connected to the inlet of the refrigerator; and     -   a second compressor having one or more inlets for receiving         evaporated refrigerant from the refrigerator and an outlet that         can be connected to the inlet of the refrigerator;

wherein the at least five outlets are intended for refrigerants evaporated at least four, preferably at least five, increasing pressures from the first outlet to the fifth and optional higher outlets.

An important advantage of the present invention is that it provides a surprisingly simple refrigerant circuit allowing the handling of five or more gaseous refrigerant streams evaporated at four or more, preferably five or more, different pressure levels in the refrigerator.

A further advantage of the present invention is that each of the first and second or even further compressors can be separately protected against overpressure, e.g. by using relief valves or the like. This may reduce the size of the pressure relief system significantly.

Another advantage of the present invention is that evaporation of refrigerant at multiple pressure levels is more efficient; the present invention allows for evaporation at more than four different pressure levels.

The person skilled in the art will readily understand that the refrigerator may have various line-ups. According to a particularly preferred embodiment it allows refrigerant to evaporate at least five different pressure levels. As the person skilled in the art understands what is meant by a refrigerator, this is not further discussed here.

The first and second compressors may be any suitable compressor. If desired, more than two compressors may be present. Also, the first and second (and even further) compressors may each comprise one or more compression stages.

According to a particularly preferred embodiment of the present invention:

-   -   the first compressor has a main inlet for receiving the         refrigerant from the first outlet, a second inlet for receiving         the refrigerant from the third outlet, a third inlet for         receiving the refrigerant from the fifth outlet and an outlet         that can be connected to the inlet of the refrigerator; and     -   the second compressor has a main inlet for receiving the         refrigerant from the second outlet, a second inlet for receiving         the refrigerant from the fourth outlet and an outlet that can be         connected to the inlet of the refrigerator.

Preferably the odd (i.e. first, third, fifth, seventh, . . . ) outlets are connected to the second compressor and the even (i.e. second, forth, sixth, eighth, . . . ) outlets are connected to the first compressor, wherein the pressure of the evaporated outlet increases from the first outlet to the fifth and optional higher outlet.

The advantage of the staggered line-up of compressor stages is that compressor powers may be evenly distributed over the first and second compressors as a result of the almost equal pressure ratios for the compressor stages.

If desired, economizers may be connected to one or more of the outlets of the refrigerator. As economizers are known in the art (see e.g. John M. Campbell, “Gas Conditioning and Processing—Vol. 2: The Equipment Modules”, 8th Edition edited by Robert A. Hubbard, 2004 page 219) this is not further discussed here. Preferably, the outlet of the refrigerator intended for the refrigerant evaporated at the highest pressure is connected to an economizer.

In another aspect the present invention provides a plant for the production of a liquefied hydrocarbon product such as liquefied natural gas, the plant comprising the refrigerant circuit according to the present invention for cooling a hydrocarbon stream such as natural gas.

In a further aspect the present invention provides a method for cooling, preferably liquefying a hydrocarbon stream to be cooled, wherein the hydrocarbon stream to be cooled is cooled using the refrigerant circuit according to the present invention.

As methods for cooling and liquefying a hydrocarbon stream are known in the art, this is not further discussed here. Examples of liquefaction processes are given in U.S. Pat. No. 6,389,844 and U.S. Pat. No. 6,370,910 which are hereby specifically incorporated by reference.

The hydrocarbon stream to be cooled and/or liquefied may be any suitable hydrocarbon-containing stream, but is usually a natural gas stream obtained from natural gas or petroleum reservoirs. As an alternative the natural gas may also be obtained from another source, also including a synthetic source such as a Fischer-Tropsch process.

Usually the hydrocarbon stream is comprised substantially of methane (e.g. >60 mol % methane). Depending on the source, the hydrocarbon stream may contain varying amounts of hydrocarbons heavier than methane such as ethane, propane, butanes and pentanes as well as some aromatic hydrocarbons. The hydrocarbon stream may also contain non-hydrocarbons such as H₂O, N₂, CO₂, H₂S and other sulphur compounds, and the like.

If desired, the hydrocarbon stream may be pre-treated before cooling. This pre-treatment may comprise removal of undesired components such as H₂O, CO₂ and H₂S, or other steps such as pre-cooling, pre-pressurizing or the like. As these steps are well known to the person skilled in the art, they are not further discussed here.

The present invention will now be described by way of example in more detail with reference to the accompanying non-limiting drawings, wherein:

FIG. 1 schematically shows a refrigerant circuit according to the present invention allowing the handling of five refrigerant streams evaporated at different pressure levels;

FIG. 2 schematically shows a refrigerant circuit according to the present invention allowing the handling of eight refrigerant streams evaporated at different pressure levels;

FIG. 3 schematically shows a refrigerant circuit according to the present invention using three compressors; and

FIG. 4 and FIG. 5 schematically show refrigerant circuits according to the present invention allowing the handling of five refrigerant streams, as alternatives to FIG. 1.

For the purpose of this description, a single reference number will be assigned to a line as well as a stream carried in that line. Same reference numbers refer to similar components.

Reference is made to FIG. 1 showing schematically a refrigerant circuit 1 containing a refrigerator (or ‘refrigeration zone’) represented by a box 2, a first compressor 3, a second compressor 4 and a cooler 5 such as an air or water cooler. Since the refrigerator 2 is well known, it is here only schematically shown for the sake of clarity.

The first and second compressors 3 and 4 arranged in separate housings. The first and second compressors in the apparatus according to the present invention may be any type of compressor, but are suitably radial compressors.

Inlet 21 of the refrigerator 2 is intended for refrigerant 10 at a refrigeration pressure. More than one inlet to the refrigerator 2 may be present.

In the embodiment of FIG. 1 the refrigerator 2 has five outlets 22, 23, 24, 25, 26 for refrigerant evaporated at different pressure levels, with increasing pressures from the first outlet 22 to the fifth outlet 26. As an example, first outlet 22 is intended for gaseous refrigerant 20 at a low pressure, second outlet 23 for gaseous refrigerant 30 at an intermediate pressure, third outlet 24 for gaseous refrigerant 40 at a high pressure, fourth outlet 25 for gaseous refrigerant 50 at a high-high pressure and fifth outlet 26 for gaseous refrigerant 60 at a high-high-high pressure.

The first compressor 3 and second compressor 4 are each arranged in a single housing. The first compressor 3 has three interconnected sections 51, 52 and 53, and the second compressor 4 has two interconnected sections 61 and 62. Each section can comprise one or more impellers, wherein an impeller is sometimes referred to as a stage. The sections 51, 52, 53, 61 and 62 are referred to as the low pressure sections 51 and 61, intermediate pressure section 52 and high pressure sections 53 and 62.

The first compressor 3 has a main or first inlet 31, a second inlet 32, a third inlet 33 and an outlet 34. The second compressor 4 has a main or first inlet 41, a second inlet 42 and an outlet 44. The main inlet 32 of the first compressor 3 opens into the low pressure section 51, and the second inlet 32 opens into the intermediate pressure section 52. The third inlet 33 opens into the high pressure section 53. The main inlet 41 of the second compressor 4 opens into the low pressure section 61, and the second inlet 42 opens into the high pressure section 62. For the sake of clarity the drivers of the compressors 3 and 4 are not shown.

The outlets 34 and 44 of the compressors 3 and 4 are connected to the inlet 21 of the refrigerator 2 by means of conduits 10, 10 a and 10 b. The first outlet 22 of the refrigerator 2 is connected to the main inlet 31 of the first compressor 3 by means of conduit 20, and the second outlet 23 is connected to the main inlet 41 of the second compressor 4 by means of conduit 30. The third outlet 24 is connected to second inlet 32 of the first compressor 3 by means of conduit 40, the fourth outlet 25 is connected to the second inlet 42 of the second compressor 4 by means of conduit 50, and the fifth outlet 26 is connected to third inlet 33 of the first compressor 3 by means of conduit 60.

During normal operation, the two compressors 3 and 4 each compress a part of the refrigerant to the refrigeration pressure, so that all refrigerant is supplied at the refrigeration pressure via conduits 10, 10 a and 10 b to the inlet 21 of the refrigerator 2. In five heat exchangers (not shown) in series the refrigerant is allowed to evaporate in the refrigerator 2.

In the first heat exchanger the refrigerant is allowed to partly evaporate at a high-high-high pressure, which is below the refrigeration pressure; the liquid part of the refrigerant is passed to the second heat exchanger and the remaining vapour is returned to the first compressor 3 through conduit 60. In the second heat exchanger the refrigerant is allowed to partly evaporate at a high-high pressure, which is below the high-high-high pressure; the liquid part of the refrigerant is passed to the third heat exchanger and the remaining vapour is returned to the second compressor 4 through conduit 50. In the third heat exchanger the refrigerant is allowed to partly evaporate at a high pressure, which is below the high-high pressure; the liquid part of the refrigerant is passed to the fourth heat exchanger and the remaining vapour is returned to the first compressor 3 through conduit 40. In the fourth heat exchanger the refrigerant is allowed to partly evaporate at an intermediate pressure, which is below the high pressure; the liquid part of the refrigerant is passed to the fifth heat exchanger and the remaining vapour is returned to the second compressor 4 through conduit 30. In the fifth heat exchanger the refrigerant is allowed to evaporate at a low pressure, which is below the intermediate pressure, and the refrigerant leaving the fifth heat exchanger is returned to the first compressor 3 through conduit 20.

If desired, economizers may be connected to one or more of the outlets of the refrigerator 2. Preferably, the outlet of the refrigerator 2 intended for the refrigerant evaporated at the highest pressure (i.e. fifth outlet 26 in FIG. 1) is connected to an economizer.

Reference is now made to FIG. 2 showing schematically a refrigerant circuit 1 according to the present invention allowing the handling of eight refrigerant streams evaporated at different pressure levels. To this end the refrigerator 2 contains 8 heat exchangers in series (not shown). Further, the refrigerator 2 has 8 outlets including sixth outlet 27, seventh outlet 28 and eighth outlet 29.

Sixth outlet 27 and eighth outlet 29 are connected (via lines 70 and 90) to third and fourth inlets 43 and 45 of the second compressor 4, while seventh outlet 28 is connected (via line 80) to the fourth inlet 35 of the first compressor 3.

In the embodiment of FIG. 2 the first and second compressors 3 and 4 have four interconnected sections 51, 52, 53, 54 and 61, 62, 63, 64 respectively.

Although it is preferred according to the present invention that the odd outlets (i.e. first outlet 22, third outlet 24, fifth outlet 26, . . . ) of the refrigerator 2 are connected to the first compressor 3 and the even outlets (i.e. the second outlet 23, fourth outlet 25, . . . ) are connected to the second compressor 4 (as is shown in FIGS. 1 and 2), the present invention also relates to alternative embodiments.

As an example, FIG. 3 shows an embodiment of a refrigerant circuit 1 according to the present invention containing more than two compressors; the refrigerant circuit 1 contains also a third compressor 6 having a main inlet 71, outlet 74 and second and third inlets 72 and 73.

The embodiment of FIG. 3 allows the handling of seven refrigerant streams 20, 30, 40, 50, 60, 70, 80 evaporated in the refrigerator 2 at four different pressure levels; a first pressure level for evaporated refrigerants 50 and 80, a second pressure level for evaporated refrigerants 40 and 70; a third pressure level for evaporated refrigerants 30 and 60; and a fourth pressure level for evaporated refrigerant 20. The pressure level decreases from the first pressure level to the fourth pressure level, i.e. stream 20 has a lower pressure than streams 50 or 80.

FIGS. 4 and 5 show examples of alternative refrigerant circuits according to the present invention also allowing the handling of five refrigerant streams evaporated at five different pressure levels, as an alternative for the line-up of FIG. 1. The pressure level at which the refrigerants 20, 30, 40, 50 and 60 are evaporated decreases from 60 to 20. It goes without saying that the lines 20, 30, 40, 50, 60 may be connected to the first and second compressors 3 and 4 in other ways; in this respect it is noted that many different line-ups may be conceived if more refrigerant streams are to be handled and if three or more compressors are used.

The person skilled in the art will readily understand that the present invention may be modified in many ways without departing from the scope of the appended claims. 

1. A refrigerant circuit, the refrigerant circuit at least comprising: a refrigerator having an inlet for refrigerant at a refrigeration pressure, and at least five outlets for evaporated refrigerant evaporated at different pressure levels; a first compressor having one or more inlets for receiving evaporated refrigerant from the refrigerator and an outlet that can be connected to the inlet of the refrigerator; and a second compressor having one or more inlets for receiving evaporated refrigerant from the refrigerator and an outlet that can be connected to the inlet of the refrigerator; wherein the at least five outlets are intended for refrigerants evaporated at at least five increasing pressures from the first outlet to the at least fifth outlet.
 2. The refrigerant circuit according to claim 1, wherein: the first compressor has a main inlet for receiving the refrigerant from the first outlet, a second inlet for receiving the refrigerant from the third outlet, a third inlet for receiving the refrigerant from the fifth outlet and an outlet that can be connected to the inlet of the refrigerator; and the second compressor has a main inlet for receiving the refrigerant from the second outlet, a second inlet for receiving the refrigerant from the fourth outlet and an outlet that can be connected to the inlet of the refrigerator.
 3. The refrigerant circuit according to claim 2, wherein the refrigerator comprises a sixth outlet for evaporated refrigerant, the sixth outlet being connected to a third inlet of the second compressor.
 4. The refrigerant circuit according to claim 1, wherein the refrigerator comprises more than six outlets for refrigerant evaporated at different pressure levels, the odd outlets being connected to the first compressor and the even outlets being connected to the second compressor.
 5. The refrigerant circuit according to claim 1, wherein the outlet of the refrigerator intended for the refrigerant evaporated at the highest pressure is connected to an economizer.
 6. A plant for the production of a liquefied hydrocarbon product, the plant comprising the refrigerant circuit according to claim 1 for cooling a hydrocarbon stream to be liquefied.
 7. A method for cooling a hydrocarbon stream, wherein the hydrocarbon stream to be cooled is cooled using the refrigerant circuit according to claim
 1. 8. The refrigerant circuit according to claim 1, in particular for use in a liquefaction plant.
 9. The refrigerant circuit according to claim 2, wherein the refrigerator comprises more than six outlets for refrigerant evaporated at different pressure levels, the odd outlets being connected to the first compressor and the even outlets being connected to the second compressor.
 10. The refrigerant circuit according to claim 3, wherein the refrigerator comprises more than six outlets for refrigerant evaporated at different pressure levels, the odd outlets being connected to the first compressor and the even outlets being connected to the second compressor.
 11. The refrigerant circuit according to claim 2, wherein the outlet of the refrigerator intended for the refrigerant evaporated at the highest pressure is connected to an economizer.
 12. The refrigerant circuit according to claim 3, wherein the outlet of the refrigerator intended for the refrigerant evaporated at the highest pressure is connected to an economizer.
 13. The refrigerant circuit according to claim 4, wherein the outlet of the refrigerator intended for the refrigerant evaporated at the highest pressure is connected to an economizer.
 14. A plant for the production of a liquefied hydrocarbon product such as liquefied natural gas, the plant comprising the refrigerant circuit according to claim 1 for cooling a hydrocarbon stream such as a natural gas stream to be liquefied.
 15. A plant for the production of a liquefied hydrocarbon product, the plant comprising the refrigerant circuit according to claim 2 for cooling a hydrocarbon stream to be liquefied.
 16. A plant for the production of a liquefied hydrocarbon product, the plant comprising the refrigerant circuit according to claim 3 for cooling a hydrocarbon stream to be liquefied.
 17. A plant for the production of a liquefied hydrocarbon product, the plant comprising the refrigerant circuit according to claim 4 for cooling a hydrocarbon stream to be liquefied.
 18. A plant for the production of a liquefied hydrocarbon product, the plant comprising the refrigerant circuit according to claim 5 for cooling a hydrocarbon stream to be liquefied.
 19. A method for liquefying a hydrocarbon stream, wherein the hydrocarbon stream to be cooled is cooled using the refrigerant circuit according to claim
 1. 20. A method for cooling a hydrocarbon stream, wherein the hydrocarbon stream to be cooled is cooled using the refrigerant circuit according to claim
 2. 